Gabriele Doblhammer and James W. Vaupel concluded that "month of birth influences adult life expectancy at ages 50+" in their paper [1] in the Proceedings of the National Academy of Sciences (PNAS).
They tracked for 30 years the mortality of all Danes who were at least 50 years old on 1 April 1968 (about 1.4 million people born before April 1918). They also analyzed about Austrians with known birth dates who died between 1988 and 1996 (about 700,000 people born before 1947) and native-born Australians who died between 1993 and 1997 (about 200,000 people born before 1948).
They found in Denmark and Austria that "adults born in autumn (October–December) live longer than those born in spring (April–June). The difference in lifespan between the spring and autumn born is twice as large in Austria (0.6 years) as in Denmark (0.3 years). ... We found the pattern in the Southern Hemisphere to be a mirror image reversal of that in the Northern Hemisphere." British born Australians were statistically closer to the Danes and Austrians.
Their analysis eliminated three hypotheses for these observations: seasonal distribution of deaths, social factors related to seasonal distribution of births, and differential infant survival. Their analysis and other studies of birth weight data led them to conclude that "seasonal differences in nutrition and disease environment early in life [in utero and infancy] could explain the relationship between month of birth and adult lifespan." Over the years, winter and spring nutrition has improved, so "the relationship between month of birth and lifespan seems to be stronger among the older birth cohorts than among the more recently born."
Hypothesis: people conceived from impulsive sex on Midsommar are more likely to have un-attending and less-caring parents, and possibly inherit lower self-control traits.
They tracked for 30 years the mortality of all Danes who were at least 50 years old on 1 April 1968 (about 1.4 million people born before April 1918). They also analyzed about Austrians with known birth dates who died between 1988 and 1996 (about 700,000 people born before 1947) and native-born Australians who died between 1993 and 1997 (about 200,000 people born before 1948).
They found in Denmark and Austria that "adults born in autumn (October–December) live longer than those born in spring (April–June). The difference in lifespan between the spring and autumn born is twice as large in Austria (0.6 years) as in Denmark (0.3 years). ... We found the pattern in the Southern Hemisphere to be a mirror image reversal of that in the Northern Hemisphere." British born Australians were statistically closer to the Danes and Austrians.
Their analysis eliminated three hypotheses for these observations: seasonal distribution of deaths, social factors related to seasonal distribution of births, and differential infant survival. Their analysis and other studies of birth weight data led them to conclude that "seasonal differences in nutrition and disease environment early in life [in utero and infancy] could explain the relationship between month of birth and adult lifespan." Over the years, winter and spring nutrition has improved, so "the relationship between month of birth and lifespan seems to be stronger among the older birth cohorts than among the more recently born."
[1] https://www.pnas.org/doi/10.1073/pnas.041431898